1. Field of the Invention
This invention relates generally to a two-step process for treating sized agglomerative coal particles preliminary to its use for making activated carbon and/or synthesis gas. More particularly, this invention relates to a process for oxidizing and heat treating agglomerative bituminous coal to render it non-agglomerative and to the resulting products.
2. Description of the Prior Art
Coal has traditionally been valued for its fuel content. More recently, there has been an increased demand for its value as a raw material for such products as activated carbon decolorents and absorbents, activated charcoal, and the like. Additionally, with increasing shortages of domestic natural gas, the demand has grown for substituting synthesis gas for natural gas using coal as the starting raw material. In the aforementioned industrial applications, or products where coal is the starting raw material, it is generally necessary to first prepare the coal for processing by conventional washing, crushing and sizing techniques. Thereafter, the coal particles are heated in an oxidizing atmosphere to an elevated temperature. For instance, where the final product is activated carbon, various procedures have been proposed employing a variety of conditions whereby the volatile material contained in the coal is distilled therefrom and either recovered for its value as a source of hydrocarbons or discarded or burned. The remaining carbon can thereafter be activated using steam or any of the other well-known activating agents.
It is already known that bituminous coal particles become plastic-like and stick together when heated to 800.degree. F., or thereabouts depending on the type of coal used, particle size, etc. This "agglomerative" effect is caused for the most part by the presence of tars and other volatiles present in the raw coal. The temperature at which the coal particles agglomerate is the "fusion temperature". This undesirable characteristic is particularly troublesome where fluidized bed reactors are employed. As particles clump and grow larger, the fluid reactor can become plugged and must be cleaned. Moreover, as the particles grow larger, it becomes more difficult to maintain the particles in a fluidized condition which is necessary for efficient reaction.
Various suggestions have heretofore been made for treating high volatile coal. For example, U.S. Pat. 3,047,472 to Gorin discloses a method for making char from coal wherein a preoxidation treatment of the crushed coal is carried out at a temperature ranging from 600.degree. F. to 850.degree. F. followed by a second oxidation at a temperature in excess of 850.degree. F.
Similarly, U.S. Pat. 3,076,751 to Minet discloses a two-step process for making char and recovering volatiles from coal. In this process, however, it is noted that an inert gas is used in a first fluid bed reactor, maintained at a temperature which can be as high as 1600.degree. F.
U.S. Pat. Nos. 3,375,175 and 3,565,766, both to Eddinger et al., disclose multi-stage fluid bed processes for pyrolyzing bituminous coal to obtain increased yields of oils and tars. As in the 3,076,751 patent, an inert gas is employed as the fluidizing medium in both the initial pretreatment and higher temperature pyrolysis. An oxidizing fluidizing medium is not employed until partial gasification in stage 4 is reached where the temperature is at least 1500.degree. F.
U.S. Pat. No. 2,805,189 to Williams discloses a process for oxidation of coal followed by carbonization. While U.S. Pat. No. 3,444,046 to Harlow suggests a process having two oxidizing steps for making metallurgical quality coke.
However, as will be apparent hereinafter, none of the aforementioned prior art techniques suggests the two-step, oxidation and heat treatment process and resulting improved product of the present invention. Nor do any of the techniques suggested by the prior art recognize the advantages which accrue from the practice of this invention.